This invention relates to the navigation of medical devices in magnetic fields, and in particular to the navigation of medical devices in magnetic fields created by magnetic resonance imaging equipment.
Systems have been developed for navigating medical devices in externally applied static magnetic fields, such as those created by magnetic resonance imaging equipment. Examples of such systems are disclosed in Kuhn, U.S. Pat. No. 6,216,026, Arenson, U.S. Pat. No. 6,304,769, and Hastings et al., U.S. Pat. No. 6,401,723, the disclosures of all of which are incorporated herein by reference. These systems employ a controllable variable magnetic moment in the medical device to orient the magnetic medical device relative to the externally applied magnetic field. One way of creating a controllable variable magnetic moment is with one or more coils in the distal end which can be selectively energized.
Although these prior systems allow effective magnetic navigation in a static magnetic field, magnetic navigation of a medical device in a static magnetic field is constrained by the nature of the interaction between the magnetic moments and the external field. The torque on a dipole moment m in a homogeneous external magnetic field B is given by τ=m×B. This torque is necessarily perpendicular to the external field B and to the moment m. In navigation it is generally possible to completely vary the direction of m relative to the catheter tip, but in fixed field navigation, such as in an MRI, B cannot be changed. Thus, when the medical device is in a plane that is perpendicular to the external field B, the device cannot be deflected in the plane in which it lies, irrespective of m, because this requires a torque that is parallel to B.
Another difficulty encountered with at least some prior systems for navigating in a static magnetic field of an MRI system is the inductive rf heating of wire leads powering the coils used to change the magnetic moment of the device.